HISTORICAL TRENDS IN FLOW DYNAMICS AND FLOOD MAGNIFICATION, TISZA RIVER, HUNGARY

This study focuses on three hydrologic measurement stations situated within Hungary: Vásárosnamény, Szolnok and Szeged. Large amounts of data (back to 1833) have been obtained from the Hungarian hydrological authority (VITUKI). This research tested the hypothesis that increased flooding on the Tisza River is the result of conveyance loss due to (1) channel and floodplain aggradation and (2) vegetation growth. Increased flood stage for a constant discharge can be caused by three and only three geometrical changes: (1) decrease in width, (2) decrease in depth and/or (3) decrease in flow velocity. In the case of the Tisza River, width during a flood event is controlled by distance between levees. Depth is controlled by aggradation or degradation of the channel and floodplain. Roughness of the channel bed and floodplain control velocity. Specific gage analysis was used to separate upstream factors from instream factors. Upstream factors are related to changes in the drainage basin that supply runoff to a river, causing a change in the volume of water (discharge) or the probability of generating a particular discharge. Instream factors are changes to the channel and floodplain that alter the conveyance capacity for a specific reach. Hydrologic data at Vásárosnamény show a decrease in the conveyance of moderate to large stages while smaller stages have remained relatively stable. Levee construction from 1991-2000 could be leading cause of this. At Szolnok, conveyance of the smallest stages has increased, representing incision. However, the conveyance of large stages has decreased despite the incision, suggesting channel roughness, not cross-sectional area, has decreased the conveyance. Szeged has seen increases in minimum depth, which seem to be the result of bridges and navigational structures. Overall, stages associated with the largest flows have increased, similar to Szolnok, confirming a regional pattern of flood magnification on the Tisza River.